Misleading study claims fast food improves exercise performance

Media reports are claiming that eating fast food could help you recover after a workout and even improve exercise performance, based on a study conducted by the researchers from the University of Montana and published in the International Journal of Sport Nutrition and Exercise Metabolism. But what did the study really say?

The research focused on glycogen, the major energy source for your skeletal muscles. Glycogen is a short-term energy supply that allows bursts of physical power; depending on the intensity of exercise, you can deplete a muscle’s glycogen stores within 10 to 30 minutes.

After exercising, it’s important to help the body restore its glycogen stores or the muscles will use less suitable energy sources, causing physiological stress and weakening the immune system. The new study asked if fast food could recharge these glycogen stores.

What did the study find?

The body makes glycogen from carbohydrates, which is why many athletes carb-load both before and after workouts: before to build up glycogen stores, and after to replenish them.

According to the researchers, prior studies have shown that other nutrients are also important for glycogen recovery. In fact, that research has very precisely delineated the ideal nutrient composition, dose and timing of foods for maximal glycogen recovery. Based in part on such research, food companies have designed a number of energy bars, shakes and other “post-workout” supplements.

In the new study, 11 male athletes worked out for 90 minutes, followed by a four-hour recovery period. During recovery, the athletes were randomly assigned to consume either sport supplements (Cliff Shot Bloks, Gatorade and organic peanut butter, plus Cytomax powder and PowerBar products two hours later) or junk food (hot cakes, hash browns and orange juice, plus a hamburger, French fries and Coke two hours later). Both groups were fed foods with the same composition of carbohydrates, fat and protein.

Before and after eating, the athletes underwent blood tests and muscle biopsies to measure glycogen levels. After eating, they also underwent a timed exercise trial. After a week, the groups were switched, and the men performed the same test but with the other group of foods.

The researchers found that regardless of which group of foods was eaten, there was no difference in glycogen recovery, blood sugar, insulin response or post-meal exercise performance.

“These data indicate that short-term food options to initiate glycogen resynthesis can include dietary options not typically marketed as sports nutrition products such as fast-food menu items,” the researchers wrote.

What do the findings mean?

The findings do not mean, as has been widely implied by media reports, that fast food is a good choice for post-exercise recovery. The study authors themselves note that the study was small and was only designed to look at specific short-term impacts. It was not designed to evaluate the long-term impacts of fast food consumption.

It’s also important to recognize how carefully planned the fast food diet in the study was. The participants did not just go to a fast food restaurant and eat whatever they wanted; instead, they were given a meal that contained the same carbohydrate, fat and protein content as the sports supplement diet. In other words, they were given a selection of fast foods designed to promote glycogen recovery.

This might not occur if people picked their own fast foods. As the researchers note in the study, many fast foods are high in nutrients that actively interfere with glycogen recovery.

If anything, the takeaway message from the study is not that fast food is good for you, but that sports supplements are not necessary for recovery after exercise. Notably, the study did not look at glycogen recovery in athletes who ate a healthy diet of real, non-processed foods.